This invention relates to a non-aqueous electrolyte cell and to a solid electrolyte cell. More particularly, it relates to a cathode and an anode representing cell components of a non-aqueous electrolyte cell and a solid electrolyte cell. Recently, with drastic progress in the art of electronic equipment, investigations into rechargeable secondary cells, as a power source usable conveniently and economically for prolonged period of time, are proceeding briskly. Known secondary cells include lead storage cells, alkali storage cells, and non-aqueous electrolyte secondary cells. A particular non-aqueous electrolyte secondary cell, namely, a lithium-ion secondary cell possesses certain advantages over other types of secondary cells including high output and high energy density. Lithium-ion secondary cells include a cathode and an anode wherein each have an active material capable of reversibly doping/undoping lithium ions. Lithium-ion secondary cells also include a non-aqueous electrolyte such as a non-aqueous electrolyte solution or a solid electrolyte.
Typical anode active materials include metal lithium, lithium alloys, such as Li—Al alloys, lithium-doped electrically-conductive high molecular weight materials, such as polyacetylene or polypyrrole, interlayer compounds in which the crystals of lithium ions are captured, and carbon materials. Typical cathode active materials include metal oxides and metal sulfides or polymers, such as TiS2, MoS2, NbSe2 or V2O5.
The discharging reaction of a lithium-ion secondary cell proceeds as lithium ions are deintercalated into an electrolyte solution at the cathode and are intercalated into the anode active material. During charging, a reaction opposite to the charging reaction proceeds, such that lithium ions are intercalated at the cathode. Thus, charging/discharging reactions are repeated as the entrance or exit of lithium ions from the cathode into and from the anode occurs repeatedly.
Known cathode active materials used in lithium-ion secondary cells include LiCoO2, LiNiO2 and LiMnO4 which are preferably used because these materials have both high energy density and high voltage. However, these cathode active materials, containing metal elements of low Clark number in their composition, suffer from high cost and difficulties met in connection with supply in stability. Moreover, these cathode active materials are higher in toxicity and have a significant negative effect on the environment. Thus, a need exists in the art for a cathode active material that does not present these disadvantages, but still possesses both high energy density and high voltage.
In response to this need, a compound having an olivinic structure and the formula LixFe1-yMyPO4 wherein 0.05≦x≦1.2 and 2.0≦y≦0.8, and wherein M is at least one element selected from the group consisting of Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and Nb, has been proposed for use as a cathode active material. This compound may be used alone or in combination with other materials. Advantages associated with the LixFe1-yMyPO4 compound include the presence of iron therein. Iron is an inexpensive material that is plentiful in supply, and is therefore less costly than LiCoO2, LiNiO2 or LiMnO4. Moreover, LixFe1-yMyPO4 is lower in toxicity and has less negative impact on the environment than LiCoO2, LiNiO2, LiMnO4 and the like.
However, when LixFe1-yMyPO4 is used as the cathode active material, and charging/discharging of the cell is carried out repeatedly, the charging/discharging capacity of the cell is decreased appreciably due to internal shorting. After the 30th cycle in a lithium-ion secondary cell wherein LixFe1-yMyPO4 is employed as the cathode active material, the electrical capacity falls to 50% or less of the initial electrical capacity. This significant decrease in electrical capacity is caused by volumetric changes that are produced in the cathode and anode as a result of cell reactions occurring in the course of charging/discharging. These volumetric changes are produced in those portions of the cathode and anode that contribute to cell reaction. In addition, those portions of the cathode and anode that do not contribute to cell reaction are also subjected to stress caused by the volumetric changes in the cell-reaction portions of the cathode and anode. This stress can result in the detachment of the active materials from the cathode and anode current collectors. This detachment, in turn, causes internal shorting of the cell device.
If, in a lithium-ion secondary cell, the cathode and anode are layered and coiled together to form a generally spirally-wound cell, the innermost portions of the cathode and anode face each other and have the same polarity. The outermost portions of the cathode and anode face an exterior material or casing. The result of this configuration is that the innermost and outermost portions of the cathode and anode do not contribute to the cell reaction.
When used as a cathode active material, LixFe1-yMyPO4 undergoes larger volumetric changes than other potential active materials. In particular, the portions of the cathode and anode that contribute to cell reaction suffer from volumetric changes that may be as high as approximately 7%. These high volumetric changes can also create larger stress on those portions of the cathode and anode that do not contribute to the cell reaction resulting in a substantial increase of internal shorting occurrences. Thus, optimum cell cyclic characteristics cannot be achieved when LixFe1-yMyPO4 is used as the cathode active material. Moreover, the electrical capacity of a cell using LixFe1-yMyPO4 as a cathode active material is smaller than that of prior art active materials and the cell size required to accommodate the active material is much larger than when prior art active materials are used. Nevertheless, LixFe1-yMyPO4 is very desirable in other respects, namely, the energy density per unit volume of LixFe1-yMyPO4 is lower than that of Co-, Ni- or Mn-based active materials. Accordingly, there is a need in the art for a secondary cell employing LixFe1-yMyPO4 as a cathode active material that does not have the disadvantages discussed above.